1. Field of the Invention
The present invention relates to a method for preparing an electrophotographic photoreceptor. In addition, the present invention also relates to a photoreceptor prepared by the method, and an image forming method, an image forming apparatus and a process cartridge using the photoreceptor.
2. Discussion of the Background
Recently, organic photoreceptors (OPCs) have been used for various image forming apparatuses such as copiers, printers, facsimiles and multi-functional apparatuses instead of inorganic photoreceptors because of having following advantages over inorganic photoreceptors.
(1) having good optical properties such that the photoreceptors have high photosensitivities over a broad wavelength range and can absorb a large amount of light;
(2) having good electric properties such as charging properties;
(3) having a wide material selectivity (i.e., various kinds of materials can be used for the photosensitive layer);
(4) having good productivity;
(5) having low costs; and
(6) having little toxicity.
Recently, image forming apparatuses are required to have a small size and to produce images at a high speed without frequently performing a maintenance operation, and therefore a need exists for a small-size photoreceptor having a good durability. In general, organic photoreceptors are soft because of having an outermost layer including a low molecular weight charge transport material and an inactive polymer. Therefore, when image forming operations such as charging, developing, transferring and cleaning operations are repeatedly performed on such organic photoreceptors, the surface of the photoreceptors can be easily abraded due to the mechanical stresses applied thereto.
In addition, in order to produce high quality images, the particle size of the toners used for forming visual images in such image forming apparatus becomes smaller and smaller. In order to well clean the surface of the photoreceptors of the image forming apparatuses, which bear residual toner particles, a cleaning blade having a high hardness is contacted with the surface of the photoreceptors at a high pressure. Thereby, abrasion of the surface of the photoreceptors is accelerated.
Abrasion of the surface of the photoreceptors deteriorates the photosensitivity and charging properties of the photoreceptors, resulting in decrease of image density and formation of abnormal images such as background development. If local abrasion is caused (such as formation of scratches) to the photoreceptors, the photoreceptors produce streak images due to defective cleaning. When the photoreceptors are thus locally or entirely abraded, the photoreceptors have to be replaced with new photoreceptors, resulting in shortening of the photoreceptors.
Therefore, in order that a photoreceptor has good durability (i.e., stable electric properties), the outermost layer of the photoreceptor has to have a good abrasion resistance. In addition, in order that a photoreceptor has good cleanability and transferability, the photoreceptor has to have good surface properties. These are problems to be solved for photoreceptors.
In attempting to solve the abrasion problem, the following techniques have been disclosed.
(1) A crosslinked binder is used for the outermost layer of the photoreceptor (published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 56-48637);
(2) A charge transport polymer is used for the photosensitive layer (JP-A 64-1728); and
(3) An inorganic filler is dispersed in the outermost layer (JP-A 04-281461).
The photoreceptor (1) using a crosslinked binder for the outermost layer thereof has a drawback in that the residual potential (i.e., the potential of an irradiated portion of the photosensitive layer) is high due to poor compatibility of the crosslinked binder resin with the charge transport material, and impurities (such as polymerization initiators, and unreacted materials and groups) included therein, and thereby low density images are produced.
The photoreceptors (2) and (3) have a relatively improved abrasion resistance. However, the abrasion resistance is not so satisfactory as to satisfy the recently required durability. In addition, the photoreceptor (3) tends to have a relatively high residual potential due to carrier traps (in general, positive hole traps are formed in OPCs) present on the surface of the inorganic filler, and thereby there is a possibility that low density images are produced.
Thus, the photoreceptors (1), (2) and (3) do not have the recently required durability (i.e., good combination of electric durability and mechanical durability).
In attempting to improve the abrasion resistance and scratch resistance of the photoreceptor (1), Japanese patent No. 3262488 (i.e., JP-A 08-262779) discloses a photoreceptor having a protective layer prepared by using a multifunctional crosslinking acrylate monomer. Although it is described therein that a charge transport material can be included in the protective layer, there is no detailed description of the charge transport material. As a result of the present inventors' study, it is found that when a low molecular weight charge transport material is included in the protective layer, the photoreceptor causes a problem in that such a low molecular weight charge transport material has poor compatibility with a crosslinked acrylate, thereby causing separation of the charge transport material from the crosslinked acrylate and formation of cracks in the protective layer. Therefore, the resultant protective layer has poor mechanical strength. It is described therein that a polycarbonate resin is included in the protective layer to improve the compatibility of a charge transport material. In this case, the content of the crosslinked acrylate is decreased, and therefore the resultant photoreceptor cannot have the desired abrasion resistance. In addition, it is described therein when no charge transport material is included in the outermost layer, the outermost layer has to be thin to decrease the residual potential (i.e., the potential of a lighted portion of the photosensitive layer). In this case, the resultant photoreceptor has unsatisfactory durability (i.e., the photoreceptor has a short life). Further, the potential of the charged photoreceptor and the potential of an irradiated portion thereof largely change depending on the environmental conditions such as temperature and humidity. Therefore, it is impossible to stably produce high quality images.
Japanese patent No. 3194392 (i.e., JP-A 05-216249) proposes another technique for improving abrasion resistance of a photosensitive layer such that a charge transport layer is formed using a coating liquid including a combination of a monomer having a carbon-carbon double bond (C═C), a binder resin, and a charge transport material having a carbon-carbon double bond, which is to be reacted with the monomer upon application of heat or light energy or a combination of a binder resin having a carbon-carbon double bond and a charge transport material having a carbon-carbon double bond, which is to be reacted with the binder resin. The photoreceptor has a good combination of abrasion resistance and electric properties. However, the monomers described therein are difunctional, and therefore the crosslinking density is relatively low. Therefore, the abrasion resistance of the photoreceptor is not so satisfactory as to satisfy the recently required durability. The same is true for the latter case where a reactive binder resin having a carbon-carbon double bond is used.
JP-A 2000-66425 proposes a photoreceptor having a photosensitive layer including a compound prepared by crosslinking a positive hole charge transport material having two or more chain-polymerizable functional groups in a molecule. However, the crosslinked positive hole charge transport material is bulky because of having two or more chain-polymerizable functional groups and therefore the photosensitive layer tends to be strained, resulting in increase of internal stress. Therefore, the photoreceptor tends to cause problems in that the surface of the photosensitive layer is roughened and/or cracks are formed in the photosensitive layer. Namely, the photoreceptor has unsatisfactory durability. In addition, since the photosensitive layer is greatly strained, the photosensitive layer has a low film density. Therefore, the photoreceptor has unsatisfactory abrasion resistance, and unsatisfactory resistance to oxidizing gases and moisture in the air, resulting in formation of abnormal images (such as ghost images). Namely, the photoreceptor cannot produce high quality images over a long period of time.
Other crosslinked charge transport layers prepared by crosslinking a radically polymerizable monomer, which has three or more functional groups and no charge transport structure, and a monofunctional radically polymerizable monomer, which has a charge transport structure, have been proposed in JP-As 2004-302450, 2004-302451, 2004-302452, 2005-099688, 2005-107401, 2005-107490, 2005-115322, 2005-140825, 2005-156784, 2005-157026, 2005-157297, 2005-189821, 2005-189828 and 20056-71856. It is attempted to impart a good combination of mechanical durability and electric durability to the photosensitive layer while preventing formation of cracks therein by using a monofunctional radically polymerizable monomer. However, depending on the preparation conditions, there is a case where the resultant charge transport layer has a low film density and does not have a satisfactory resistance to oxidizing gases and moisture. In this case, abnormal images (such as ghost images) tend to be formed. In addition, depending on the preparation conditions, there is a case where the charge transport structure of the monofunctional radically polymerizable monomer is damaged by light applied thereto to crosslink the monomers, resulting in deterioration of the charge transport function (i.e., deterioration of image qualities).
Because of these reasons, a need exists for a photoreceptor which can maintain a good combination of electric properties and mechanical durability and which can produce high quality images over a long period of time.